Known in the art is an arrangement which distributes fuel between the main and the pilot manifolds of a gas-turbine engine and is controlled, for example, by compressor output air pressure. Such arrangement comprises a needle valve incorporated in a passage connecting the metering element delivery line to the main fuel manifold.
The main and pilot manifolds are connected respectively with vaporizing-type main burners and swirl-type pilot burners in the engine combustion chamber. The needle valve is connected through a mechanical linkage to a bellows arranged to receive air from the engine compressor.
During the initial period of starting the gas-turbine engine the passage conveying fuel from the metering element to the main manifold is shut off by the needle valve and the entire fuel supply from the metering element is delivered into the pilot manifold from where it is injected through the swirl-type pilot burners into the combustion chamber. Fuel combustion raises gas temperature at the entry to the turbine, which causes an increase in engine speed during the initial starting period. Under these conditions the compressor output pressure and, consequently, the pressure inside the bellows increases. The bellows extends and partially opens the needle valve, whereby the fuel supply from the metering valve is allowed to pass into the main manifold and the vaporizing-type burners of the main manifold discharge the fuel into the combustion chamber. As the compressor output pressure increases, the needle valve opens further and the supply of fuel from the metering element to the main manifold increases.
The arrangement described above suffers from the disadvantage that it has no provision for regulating the fuel pressure in the pilot manifold and the minimum rate of fuel flow in the main manifold is difficult to control. This causes high pressure build-up in the pilot manifold, which results in local overheating of the combustion chamber flame tube and non-uniformity of the combustion chamber temperature field, whereby the engine fails to start surely.
An arrangement is known in the art which partially eliminates these disadvantages. In this arrangement a valve is provided in the passage connecting the metering element delivery line to the main manifold, and a fuel jet is provided in the passage connecting the metering element delivery line to the pilot manifold. Said valve maintains the fuel pressure in the metering element delivery line depending on the fuel pressure differential across the jet. However, the fuel pressure in the metering element delivery line varies with alteration of the rate of fuel flow resulting, for example, from changes in the engine operating conditions (such as speed and altitude of flight). Furthermore, there is no provision for fuel to go past said jet and valve incorporated in the passages connecting the metering element delivery line to the main and pilot manifolds, which condition results in pressure build-up in the fuel pump and consequent shortening of the pump life.
It is known that variation in ambient air temperature affects the fuel atomizing performance of swirl-type burners. The fuel distributing arrangements known in the prior art do not cater to this condition and therefore they adversely affect fuel atomizatiOn at low ambient temperatures or cause gas overheating at the entry to the turbine at high ambient temperatures.